The present invention relates to the methods which make it possible to produce contacts between two conductive layers separated by an insulating layer.
More particularly, the invention concerns a simple, quick and very low cost method of producing ohmic or thermal contacts between two conductive layers separated by a less conductive layer.
Contact, in the context of the following description, must be taken to mean any area of connection between two layers which preferentially allows the passage of a current (ohmic contact) or heat (thermal contact) between these two more conductive layers separated by an intermediate layer of lower conductivity, the set of all the layers being referred to hereinafter as the system and the intermediate layer being referred to hereinafter as the support.
The more conductive layers can for example be metallic multilayers, or films, or metallic patterns, made of aluminium, copper or an alloy.
The intermediate layer can for example be an adhesive film, a dielectric, an insulating film, an extruded layer, a polymer or any other material having dielectric and/or conductive properties, any layer of lower resistivity providing insulation between the two more conductive layers, or a set of a number of layers of this type.
The present application more particularly concerns the double-sided flexible printed circuit type multilayer systems which comprise metallic patterns on either side of a support dielectric, it being necessary to connect said patterns ohmically at certain points in order to provide the functionality of the circuit.
The simplest contact methods, for example by stamping, just consist of a physical (mechanical) contact between the conductive layers.
These quick and inexpensive methods are generally sufficient for systems which are not greatly stressed mechanically or those requiring a low reliability or little constrained as regards thickness, or mechanically robust, operating at low current intensity, or at high frequency, or whose functionality tolerates variation in the ohmic characteristics of the contact, or whose composition favours the relative good quality of the contact.
The contact is produced by tearing of the support and deformation of the layers. This is for example the case of the method of producing contacts used in the antitheft circuits which are found in stores. The dielectric is highly elastic and has contraction effects favourable to maintaining the physical or proximity contact of the metallic films.
Among the more complex and costly methods, there can be cited the case of the mechanical methods of contact by riveting which consist in adding a part which will make the mechanical connection by clamping. The contact is produced by tearing of the support and contact is made by an added element.
For the most complex, and the most costly, methods, contacts of good ohmic quality are obtained, but at the price of a method which is elaborate and constraining as regards the chronological sequence of the steps constituting the system itself, and the choice of materials for this system. The contact is produced by opening of the support and formation of intermetallic compounds. The principle of these methods is adhesive force.
This is for example the case of semiconductor or hybrid multilayer type contact methods, which consist in opening vias in the intermediate support of the system, these vias opening out on to the first metallic layer, and in there depositing a metallic alloy constituting the second conductive layer of the system, these steps often being supplemented by a heat treatment allowing optimization of the intermetallic connection in the via.
Among these complex and costly methods, there are also those which make it possible to obtain contacts of good ohmic quality on contactless prefabricated systems, but at the price of additional, often costly and difficult, steps and a supply of additional intermediate material making it possible to provide the inter-layer connection, the performance of this connection, its preservation over time and its reliability. The contact is produced by cutting of the support and addition of an intermediate material. The principle of these methods is gluing.
This is for example the case of the methods of manufacturing contacts in printed circuits, which consist in opening vias in the support separating the conductive layers and in there depositing a metallic alloy forming the contact; the method often being supplemented by a heat treatment allowing optimization of the intermetallic connection between the conductive layers. There will also be cited for example the solutions used for the most recent flexible printed circuit systems, for which cutting is used for opening vias, and conductive glues are used as intermediate material, the overflows of this material providing the contact.
A major problem of the xe2x80x9cmechanical contactxe2x80x9d methods or those of the xe2x80x9cintermetallic compound contactxe2x80x9d type is the impossibility of simultaneously obtaining good ohmic contacts and very low production costs, essential for certain applications or for certain products.
The weakness of purely mechanical contacts lies in the transience of the contact, dependent on the quality of the stamping or the xe2x80x9crivetingxe2x80x9d. Pressure is the basic principle of these methods. The presence of an intermediate initially with no via reduces the quality of the contact. Environmental constraints, like for example a subsequent lamination, are also detrimental to the reliability of these contacts. Finally, the choice of the support material is one of the limiting factors and the key to the reliability of this type of contact.
In the prior art, the low cost/performance duality of the contact was not accessible until now.
The invention proposes a production method making it possible to obtain good reliable contacts on multilayer systems having at least two conductive layers separated by at least one intermediate layer of lower conductivity (like for example an insulating support), while keeping very low production costs.
The object of the present invention is more particularly a method of producing at least one contact between two conductive layers separated by an insulating layer, principally characterised in that the complex formed by these layers is pressed between a means excited by ultrasound applied perpendicular to the face of the complex, and a fixed means whose face resting on the complex has at least one protruding part capable of deforming the complex during application of the ultrasound in order to allow flowing and opening of the insulating layer in the region of each protruding part, and then bringing into contact and welding of the metallic layers in the region of this same protruding part.
According to another characteristic, pressure is maintained between the means excited by the ultrasound and the fixed means after stopping of the application of the ultrasound which allowed the flowing of the insulating layer, until the insulating layer has cooled and solidified.
According to another characteristic, the surface of the fixed means bearing each protruding part has radial triangular channels joined by protruding ridges.
According to another characteristic, the method consists in transferring the complex between a delivery roller and a receiver roller, the fixed means having the form of a rotary roller whose axis is parallel to that of the delivery rollers and winders and whose lateral face is etched and turns at the same speed adapted to that at which the complex travels past.
The invention brings together the advantages of the mechanical methods, since no material is supplied and the method is quick, and the advantages of the intermetallic compound contact methods, since it results in a metallic interpenetration of the layers to be connected.
The invention, particularly well adapted for keyboards produced with flexible printed circuits, or for micro-electronic chip transponders based on printed circuit type resonant circuits, applies to any type of system.
The invention opens possibilities of simultaneous and direct contacts between electronic devices and conductive film systems.
The object of the invention is a method of localized shearing of the system to be provided with contacts, followed by heating and the formation of an ultrasonic weld type contact in the areas where the conductive layers are in physical contact. The form of the tool and its material are carefully designed to allow all the following: cutting, heating of the support, bringing the conductive layers situated on either side of the support to face one another, and their welding.
Preferentially, the system has a thermoplastic support of thickness less than 100 micrometers (typically 25 micrometers) and metallic patterns made of aluminium, copper, or metallic alloy, of thickness less than a few tens of micrometers (typically between 10 and 50 micrometers), situated on either side of the support.
In a preferred embodiment, the method is implemented on a single item of equipment which makes it possible to produce a number of contacts simultaneously, in step by step mode. A variant embodiment consists in producing the contacts continuously on roller or cut sheet systems.
The production of welded contacts through an insulating support has many advantages, making it possible to eliminate all or some of the problems cited previously. This is because, in this case, the contact is completely ohmic (resistance less than 0.1 milliohm), and mechanically resistant to lamination, thermal cycling, and the mechanical constraints of the products cited above (keyboard, etc.).
Thus, the problem of contact reliability is solved.
Another advantage is the fineness of the contact, the localized deformation of which can be reduced to minimum values, less than for example 200 micrometers.
In a more complex case, the multilayer system has two metallic layers, for example made of aluminium, separated by an insulating layer, for example made of PET.